(1) Field of the Invention
The present invention relates to a color image forming apparatus such as a color printer, etc., and relates to a so-called tandem type color image forming apparatus wherein a multiple number of photoreceptors are charged so as to develop color images by developing devices holding different color toners.
(2) Description of the Prior Art
Recently, in the field of color electrophotographic processing, tandem type color image forming apparatuses in which a multiple number of photoreceptor drums for multiple colors of toner are arranged in line to obtain a color image have been used in order to enhance the printing speed. This tandem type configuration lends itself to color image forming apparatuses and multi-color image forming apparatuses for outputting image formed articles of reproduction and composition of color images and multi-color images by successively transferring a plurality of color separation images for color image data or multi-color image data, in a layered manner, as well as image forming apparatuses including a color image forming function or multi-color image forming function. It is essential for these image forming apparatuses that all the photoreceptors arranged therein should always have the same level of quality in order to provide images without color imbalance between the color components.
Even if uniform images free from unevenness can be obtained when all the photoreceptors are unused, the problem may take place that the image quality becomes degraded as the photoreceptors are worn down as they are used. Despite of its name, a color image forming apparatus in practice is often used for monochrome (black/white) printing other than color printing. There are cases where monochrome printing is implemented more often than color printing, hence there is a drawback that the photoreceptor for black images becomes worn away earlier than the other color photoreceptors.
Usually, the processing system is designed so that the four photoreceptors for the four colors Y, M, C and K(Bk) toners will not present inharmonious wear characteristics. If, however, the photoreceptors for individual toners are worn away in different manners, there occurs color unevenness and color imbalance as the number of copies increases. In such cases, all the drums, instead of the drum which, alone, has been heavily degraded, should be replaced. Particularly, if hard papers such as post cards are used, large wear takes place locally, causing large influences.
Further, when contact type chargers which will impose heavier burdens on the photoreceptors are used, the amounts of wear of the drums become large. If the wear of the photoreceptor is made small and uniform, it is possible to make the interval for replacement of the drum longer. Further, if all the drums reach the end of their life at almost the same time, concurrent replacement of all the drums will never produce any loss. However, if the wear and degradation rates of the drums differ between different colors of developing devices, degradation of only one of them requires replacement of all the drums. Otherwise, color imbalance between the new drum and the other drums, which have not been replaced, takes place, resulting in failure to obtain good image quality. In other words, the interval of drum replacement is determined by the most intensively degraded drum among the four. This results in being wasteful and uneconomical.
As countermeasures, Japanese Patent Application Laid-open Hei 10 No.333393, Japanese Patent Application Laid-open Hei 11 No.24358 and Japanese Patent Application Laid-open Hei 11 No. 52599, disclose configurations in which an α-Si or α-SiC photoreceptor is used for that for black development so as to enhance the photoreceptor life while OPCs (organic photoreceptors) are used for those other than that for black development. There is, however, a problem that α-Si and α-SiC photoreceptors used in the above publications are less chargeable. As a solution to this drawback, Japanese Patent Application Laid-open Hei 10 No.333393 specifies the thickness of the photoreceptor to be 30 μm or more and its difference in surface potential from the other organic photoreceptors to be equal to or lower than 200 V. Japanese Patent Application Laid-open Hei 11 No.24358 proposes that the applied voltage to the α-Si photoreceptor should be 1.05 to 2.50 times the application voltage to the organic photoreceptors. Further, Japanese Patent Application Laid-open Hei 11 No.52599 is aimed at increasing the chargeability by adding an α-SiC surface layer.
In the above way, in order to extend the life of the photoreceptor for black development while making up for the low chargeability of the α-Si or α-SiC photoreceptor, it is necessary to make complicated charge control for black development, resulting in the need of extra cost. Further, since, other than the charge control, there are differences in light sensitivity and susceptivity to temperature/humidity, between the α-Si or α-SiC photoreceptor and the organic photoreceptor, light exposure, transfer conditions and other factors differ between the α-Si or α-SiC photoreceptor for black development and the organic photoreceptors for development other than black. Therefore, a different control method of the photoreceptor for black development from that for the photoreceptors for the other colors should be used, thus again resulting in the need of extra cost. The α-Si or α-SiC photoreceptors disclosed in Japanese Patent Application Laid-open Hei 10 No.333393, Japanese Patent Application Laid-open Hei 11 No.24358 and Japanese Patent Application Laid-open Hei 11 No.52599, have the problem that their production cost is obviously high compared to the organic photoreceptors. Further, as another problem, they consume large amounts of black toner, as is well known.
As the countermeasures against the above problems, Japanese Patent Application Laid-open 2000 Nos.242056 and 242057 propose configurations where the drum for black development alone is increased in diameter or increased in film thickness. Japanese Patent Application Laid-open 2001 No.51467 refers to use of a non-contact type charging means only for black development, increase in film thickness and use of a resin having a large viscosity-average molecular weight. Further Japanese Patent Application Laid-open 2000 No.330303 discloses a polycarbonate copolymer resin as the resin for tandem photoreceptors. Further, provision of a protective layer on only the photoreceptor for black development has been also investigated as an optional method.
Increase of the drum diameter for black development alone as in Japanese Patent Application Laid-open 2000 Nos.242056 and 242057 results in enlargement of the machine body. Increase in thickness of the coating film may cause reduction in the amount of charge or degrade dot reproducibility and/or line reproducibility in the image. Further, use of a resin having a large viscosity-average molecular weight produces an air entrapment problem when it is applied and causes difficulties in application. Japanese Patent Application Laid-open 2000 No.330303 also discloses use of various copolymer polycarbonate resins as the resin for tandem photoreceptors and refers to the relationship between the maximum/minimum abrasion losses. However, the discussed photoreceptors for black and other color development use an identical configuration, hence it is impossible to lengthen the life of the photoreceptor for black development in a general environment in which monochrome copy mode is used often.
On the other hand, Japanese Patent Application Laid-open 2001 No.249576 refers to increase in film thickness of the photoreceptor layer in order to improve the abrasion resistance of the photoreceptive layer of the photoreceptor used in the image forming and transfer unit undergoing a greater contact abrasive force. However, when, for example, a silicon photoreceptor presenting a markedly large abrasion resistance is used for black development only, the photoreceptor for black images, alone, is still usable despite the photoreceptors for colors having already reached the end of their life, bringing about a reversal in the relationship, so this cannot be said to be the perfect solution.